1. Field of the Invention
The present invention relates to a method and apparatus for conducting liquid-liquid extraction. More specifically, the present invention utilizes a liquid-liquid contactor provided with electrodes that are capable of being electrically energized to establish an electrostatic field within the contactor. The electrostatic field functions to mix an aqueous phase with an organic phase and to separate the two phases without any other means of liquid dispersers, mixers or settlers.
2. Description of the Related Art
In separation processes dealing with liquid drops, various attempts have been made to enhance the mass transfer efficiencies of the drops. Generally, transfer efficiency is enhanced by producing a larger interfacial area and a higher degree of turbulence within and around drops to increase eddy diffusion. The requirement for creating local turbulence coupled with a large interfacial area is difficult because these features are incompatible in the sense that small drops do not have high relative velocities nor do they exhibit marked internal circulation patterns. The present invention utilizes an electric field as a technique to overcome these problems. This technique has the advantage of easily producing small charged drops by use of electrostatic force, and the charged drops can move through an extracted organic phase with higher velocity due to the Coulomb force, which movement induces a high degree of fluid turbulence around and within the drops. This technique can be applied to mass transfer operations, such as metal extraction, to produce high interfacial areas coupled with enhanced mass transfer co-efficients.
Furthermore, direct utilization of electrical energy in processes involving liquid drops will give higher energy efficiency than energy supplied in the form of thermal or mechanical energy. Also, by eliminating the need for mechanical mixing apparatus, capital and maintenance costs can also be reduced.
Many electrostatic liquid-liquid contactors have been proposed. For example, U.S. Pat. No. 4,161,439 to Warren et al teaches a contactor for solvent extraction for use in the mining industry. Warren teaches use of settling tanks to separate the organic and aqueous phases which are mixed in the Warren contactor. Output of this type of process has been limited by the size of the settling tanks and residency time required in the settling tanks to obtain satisfactory separation.
The present invention obtains improved settling rates by using the electric field to separate the aqueous and organic phases. The use of this invention is particularly applicable to solvent extraction, extractive fermentation, and desalting of crude oil.
Recovering and purifying biochemical products from complex fermentation broth mixtures is one branch of selective solvent extraction for which the present invention is useful. Such biochemical products include new pharmaceuticals, secondary metabolic antibiotics, polymer intermediates, thickeners and modifiers, and enzymes, etc. The present invention offers the fundamental advantage of high selectivity and operation at ambient temperatures which is an important consideration for the recovery of heat sensitive molecules.
Any separation process for use in recovering products from fermentation liquors faces not only the problem of product stability, but also the physical properties of the broth itself, specifically, the problem of removing dead micro-organisms, cell debris, and lysis material prior to separation. This solid/liquid separation can be a major economic constraint in terms of cost of separation equipment and lost recovery. High viscosity and non-Newtonian behavior of some fermentation broths are a further difficulty encountered particularly when using conventional liquid-liquid extraction methods. Also, the solid materials can form stable emulsions or collect at the interface and significantly reduce mass transfer efficiencies.
In the area of extractive fermentations, there has been a strong interest recently in developing solvent extraction technology for recovery and purification of fuel alcohol arising from fermentation processes. The energy intensive nature of distillation processes and the cost involved can represent a substantial portion of the value of the alcohol product. Also, another common property of many biological product systems is that maximum yield of product is subject to a phenomena of product inhibition whereby the activity of the micro-organisms involved in the fermentation is suppressed by the increasing concentration of the fermentation product. At higher concentrations of the fermentation product, the micro-organism activity is suppressed to unacceptably low levels, thus, placing a limit upon the final product concentrations achievable in a batch process. The present invention can be used as an in-line technique for removing fermentation products from batch fermentation processes, allowing significant improvements in the maximum achievable yield and conversion and, thereby, ameliorating the effects of product inhibition. Also, as a side benefit, the voltage applied to the microorganisms can by electrical stimulus increase their activity.